The objective of this proposal is to determine the functional group requirements for the potent inhibition of pepsin and R. chinensis pepsin by applying detailed quantitative kinetic analysis of inhibition to synthetic analogs of pepstatin. The kinetic analysis involves distinguishing between a diffusion controlled dissociation constant (k2/k1) and a slower tightening of inhibitor binding, which give rise to extremely small net inhibition constants and to time-dependent onsets of inhibition. The pepstatin analogs to be synthesized and kinetically examined fall into the following categories: a. presence, number and position of hydroxyl groups; b. position and identity of hydrophobic groups; c. increasing and decreasing chain length at both carboxyl and amino terminii; d. substitution with novel amino acids. A total of 18 analogs of pepstatin are proposed for eventual kinetic analysis. Of these 8 have already been synthesized and subjected to kinetic analysis in this laboratory. Of the remaining 10 compounds, the synthesis of 6 statine containing analogs is assured by known methods leaving 4 compounds to be prepared from established amino acids by proposed methods. Systematic correlation between structure and kinetic parameters and collaborative X-ray crystalography are expected to provide precise information of both the inhibition and mechanism of action of pepsins, that will be significant to the development of new therapeutic agents for the treatment of hypertension gastric ulcers, inflammatory disease and other diseases in which acid proteases are involved.